This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9902209-7 filed in Sweden on Jun. 11, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to preventive performance testing for a radio system comprising a transmitter and a receiver and more specifically to in-service measurements of the fade margin in a radio relay system.
Radio relay systems or radio links work with a certain fade margin between the received input power level and the receiver detection threshold in order to encounter various conditions affecting the reception of the transmitted signal. For instance, the transmitted signal may be subject to varying climatic attenuation due to rain or due to multi-path propagation caused by refractivity gradients in the atmosphere. Moreover, varying interference signal levels from other radio sources may influence the detection threshold negatively.
If the received signal is falling under the detection threshold, a traffic interruption may occur. Over a given period of time, these interruptions can be described by means of the cumulative outage time. Typically, network providers require such a high quality of service that the cumulative outage time should not exceed a limit, which could be as low as 10 minutes per year.
The designer of the radio relay system is faced with the task of providing a link, which fulfils these requirements. Moreover, the radio relay system must comply with current regulations on radio emission values, which applies for a given site and frequency band, whereby the output power of the radio relay system may not exceed a certain limit.
The radio link should be dimensioned having a sufficient fade margin in order to achieve reliable performance. The fade margin may for instance be expressed as the amount of extra output power which should be available under normal clear sky conditions in order to guarantee a given minimum bit error rate threshold for the received signal under worst case conditions.
Therefore, before the radio link can be taken into operation an estimation of worst case conditions must be made such that the necessary fade margin can be found.
In the following, a prior art radio link as well as known dedicated measurement equipment used in connection with the installation shall be explained together with a known procedure for measuring the fade margin. Reference is made to the block diagram of the prior art apparatus shown in FIG. 1.
The radio link comprises the following elements on the transmitter side: A modulator, MOD typically comprising forward error correction encoding means, EM1, and having an input terminal to which the input signal Si is provided, a power amplifier, AMP coupled to the modulator MOD, a transmit antenna, TXa. Optionally, the path between the power amplifier AMP and the transmit antenna TXa can be disconnected for measurement purposes.
The radio link comprises the following elements on the receiver side: A receiving antenna, RXa, a reception signal level meter, RSSI, providing a numerical value indication of the reception level, an automatic gain control element, AGC, and a demodulator, DEM, typically comprising forward error correction decoding means, EM2, and providing at its output terminals the forward error corrected signal, So. As is well known, the use of forward error correction (FEC) implies that a certain level of the signal content being transmitted is redundant. The forward error correction coding renders the system more robust such that, even if the signal level falls under the detection threshold for a certain period of time, it is possible to restore the original content of the code.
A measurement equipment comprising the following elements is used in connection with the installation of the radio link described above: A pattern generator, PG, is shown coupled to the signal input at the modulator, MOD and signal analyser, SA, is coupled to the output side at So. From an analysis of the generated signal and the received signal, the bit error rate can be calculated.
As shown in FIG. 1, the physical effect of antenna/air interface is represented by function block AIR, while contributions from surrounding radio sources have been indicated by means of source N(t).
During measurements, an attenuation block (not shown), can be inserted in the signal path between the amplifier AMP and the transmit antenna TXa at point At in order to attenuate the signal thereby simulating the attenuation which would be caused by bad weather.
When the projection and physical installation has been completed for a given radio link, the measurement equipment described above is coupled to the equipment and fade margin tests can be undertaken.
By attenuating the transmitted signal, under stable atmospheric conditions, the bit error rate of the output signal of the demodulator can be mapped as a function of the signal attenuation or input signal strength. These measurements are performed taking into account the current noise level from other sources.
The installation/fade margin test process can be described as iterative, whereby system design parameters, such as antenna configuration and placement, are changed until these values match the required fade margin.
In prior art document WO96/31009 a two way adaptive power control for a two way link has been described in which the signal deterioration measured in terms of a bit error rate on each respective link are taken into account for regulating the power emission.
The transmitter/receiver system according to the above document comprises a feedback path in which the output power of the transmitter is regulated in correspondence with the input power measured by the receiver or in correspondence with the signal to noise ratio of the signal measured by the receiver.
However, one problem with the system described according the above document is that the user of such a system always transmits with the lowest transmitter power level that ensures error-free transmission. This user will not gain any benefits from this mechanism; only other users in the vicinity will gain. Moreover, it is difficult to prove that a stable system can be achieved in a complex environment with hundreds of users with mutual interference paths. The result is that users will not choose such a system, since mainly others, who are typically competitors, will gain.
As follows from the above description, the known fade margin test process is time and manpower consuming. Moreover, fade margin tests can only be applied prior to the actual operation, because no traffic can be sent on the link when the measurement equipment is inserted in the signal path.
In a steadily changing situation where new radio sources are continuously installed in the environment, it is of course difficult to assess the actual fade margin. Over time, the noise level may increase so much that the fade margin implemented at installation may prove to be insufficient.
Therefore, it is a first object of the present invention is to accomplish a performance test for a radio system and more specifically an in service fade margin test for a radio relay system, in order to ensure that the required fade margin is prevalent.
This object has been achieved by the subject matter set forth in independent method claim 1 and independent apparatus claim 9.
The test according to the invention is advantageously performed during a short period of time without interrupting the traffic messages being sent on the link.
It is another object to accomplish a performance test based on monitoring the output power level of the transmitter.
This object has been achieved by the subject matter defined by claim 2.
It is another object to achieve a performance test based on monitoring the input power level of the receiver.
This object has been achieved by the subject matter defined by claim 3.
The fade margin test according to the invention is either initiated when desired or initiated automatically by clock-initialisation, i.e. while the path is operating in a traditional way.
Further advantages will appear from the remaining claims and the following detailed description of the invention.